Examples of direct-current power supply devices include an AC-DC converter which is structured by a diode bridge circuit that rectifies an alternating-current power supply and an insulated DC-DC converter that steps down the direct-current voltage rectified in the diode bridge circuit to convert the voltage into a direct-current voltage with a desired potential. One known example of such an AC-DC converter is a switching power supply device in which a switching element connected in series with a primary-side winding wire of a transformer for voltage conversion is turned on or off by a PWM (pulse width modulation) control method, aPFM (pulse frequency modulation) control method, or the like, to control the current flowing in the primary-side winding wire, so that the voltage induced by a secondary-side winding wire is controlled indirectly.
In some switching-control type AC-DC converters, for the control operation on the primary side, a resistor for current detection is provided in series with a switching element on a primary side and moreover a power supply control circuit (IC) is provided with a terminal to which the voltage obtained by current-voltage conversion in the resistor is input (current detection terminal). In this AC-DC converter, based on the detected current value and a feedback voltage from the secondary side, the peak current in the primary winding wire is controlled to maintain the output voltage or the output current on the secondary side constant (see Patent Literature 1).